A surprisingly gentle merger between two small primordial bodies formed the distant object Ultima Thule, a new study suggests.
These two progenitors themselves likely coalesced from the same cloud of icy material at the dawn of the solar system, billions of miles from the newborn sun. They initially circled a common center of mass but spiraled closer and closer, eventually meeting up in decidedly leisurely fashion.
"These guys look like they came together at literally spacecraft-docking speed," said study lead author Alan Stern, of the Southwest Research Institute in Boulder, Colorado. "This really is informative about the origin of planetesimals out there."
Stern is principal investigator of NASA's New Horizons mission, which flew by Ultima Thule on Jan. 1 of this year. The new study, which was published online today (May 16) in the journal Science, describes the initial science returns from that flyby, the most-distant planetary encounter in the history of spaceflight.
Distant target number 2
New Horizons launched in January 2006 to get the first-ever up-close look at Pluto, which had remained largely mysterious since its 1930 discovery. The spacecraft checked off this main mission goal in July 2015, zooming within 7,800 miles (12,500 kilometers) of the dwarf planet and capturing amazing images of its stunningly complex and diverse surface.
The New Horizons team then turned its attention to Ultima Thule (official name: 2014 MU69), a small object that currently lies about 4 billion miles (6.5 billion km) from Earth — 1 billion miles (1.6 billion km) more distant than Pluto.
The Jan. 1 flyby, the centerpiece of New Horizons' extended mission, was an even more challenging spaceflight feat than the Pluto encounter.
Ultima Thule is much smaller than the dwarf planet, spanning just 22 miles (35 km) in its longest dimension. And New Horizons gave Ultima Thule a much closer shave than it did Pluto, cruising a mere 2,200 miles (3,540 km) above the small object's frigid surface. The probe was barreling along at 32,280 mph (51,950 km/h) relative to its target at the time.
So far, New Horizons has beamed home just 25% of its flyby data, which should all be in hand by mid-2020. The new study is based on just 10% of the expected total haul — the amount available when the researchers submitted the paper in late February, Stern said. But even this limited first look at New Horizons' imagery and measurements has produced some very intriguing results, as the new paper shows.
A flattened, reddish snowman
New Horizons found that 2014 MU69 is a "contact binary" composed of two lobes, which the team dubbed Ultima (the bigger one) and Thule. The object therefore looks like a snowman — a crushed and bloody one, anyway.
Ultima Thule is the reddest object ever explored by a spacecraft, with the exception of Mars, Stern said. The Red Planet owes its hue to iron oxides (rust), but something else is going on with 2014 MU69. The New Horizons team thinks the color comes from complex organic molecules known as tholins, or something like them.
This would not be unheard of; tholins are thought to be responsible for the reddish swaths that New Horizons spotted on Pluto and its biggest moon, Charon.
And then there's the crushed bit: Ultima Thule, especially the Ultima lobe, is notably flattened — "something that really no one expected or predicted with models, sending the theorists back to the drawing board," Stern told Space.com.
The team isn't sure how Ultima Thule got its pancake-esque shape. It's possible that rapid rotation played a role, Stern said. (The object currently completes one spin every 15.9 hours, but the two constituent lobes may have rotated much faster in their youth, before the merger.)
"Or maybe there was a lot of aerodynamic erosion," Stern said, invoking the possibility that gas and grains of material that didn't get incorporated into Ultima or Thule could have scoured them down.
Many other mysteries remain as well. For example, Ultima sports a number of similar-sized abutting mounds, which may be the outlines of the smaller pieces that built up the lobe. No such mounds are visible on Thule, however.
That may be because the two lobes formed in slightly different ways. But Thule has a 4.3-mile-wide (7 km) crater called Maryland. So, it's possible that the lobe had mounds as well, but these features were buried when the Maryland-gouging impact resurfaced Thule, Stern said. (New Horizons did not spy any big craters on the Ultima lobe.)
In addition, both lobes have numerous small pits, whose origin remains undetermined. And multiple formation processes are likely involved, study team members said.
"Our assessment is that the chains of similarly sized pits are more likely to be formed by internal processes than by cratering, but the isolated pits that show approximately circular planform outlines, bowl-shaped interior depressions, and, in some cases, raised rims, are more consistent with impact crater morphology," the researchers wrote in the new study.
The New Horizons team has not yet spotted any satellites or rings orbiting Ultima Thule, and the object has shown no signs of an atmosphere or any comet-like outgassing. But the researchers will keep looking as more and more data comes down to Earth.
A primordial object
Ultima Thule's two-lobed shape strongly suggests that the object is primordial, going all the way back to the solar system's birth.
Impact speeds in 2014 MU69's neighborhood — the cold, dark depths beyond Neptune known as the Kuiper Belt — are currently around 670 mph (1,080 km/h). A modern meetup of two objects out there would thus be too violent to produce the Ultima Thule we see today; its two lobes would have been destroyed or misshapen, Stern and his colleagues found.
Indeed, modeling work the team presented at a conference last month suggests the collision likely occurred at around 5.5 mph (8.9 km/h) — slower than most joggers go. Such a "gentle dynamical environment" was present long ago, shortly after the sun had formed.
Other lines of evidence bolster the notion that Ultima Thule is an ancient and relatively unchanged object. For example, its two lobes are similar in both brightness and color, suggesting that they formed from the same swirling cloud of gas and dust long ago.
"This is the first unquestionably primordial contact binary that we've seen up close with a spacecraft," Stern said.
More work to do
New Horizons team members may end up cracking more Ultima Thule mysteries one day; most of the flyby data still hasn't come down to Earth, after all.
And even when that information has all been analyzed, there may be yet more work to do. The spacecraft is healthy and has enough fuel left to fly by yet another deep-space object, Stern said.
NASA would have to grant another mission extension for this to happen, and the New Horizons team can't apply for such an extension until next year, Stern said. But the researchers definitely plan to do so.
"We came all the way out here to the Kuiper Belt, and we're going to try to squeeze every last thing we can [out of this mission]," Stern said.
More spacecraft will explore the outer solar system in the future, "but they're not going to be here anytime soon," Stern said. "We're here, and we're going to milk it."
- New Horizons' Historic Flyby of Ultima Thule: Full Coverage
- Destination Pluto: NASA's New Horizons Mission in Pictures
- New Horizons May Make Yet Another Flyby After Ultima Thule
Mike Wall's book about the search for alien life, "Out There (opens in new tab)" (Grand Central Publishing, 2018; illustrated by Karl Tate), is out now. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook.